Mist eliminators have found widespread use in applications wherein aerosols, particularly of less than 3 microns, must be separated from a gas or vapor (hereinafter and in the claims collectively referred to as “gas”) stream. These mist eliminators include fiber beds through which the gas stream is passed to achieve separation. Some of the more frequent applications of mist eliminators include removal of acid mists, such as sulfuric acid mist, in acid manufacturing processes, plasticizer mists in, for example, polyvinyl chloride floor or wall covering manufacture, water soluble solid aerosols such as, for example, emissions from ammonium nitrate prill towers. In removal of wetted soluble solid aerosols, the collected solid particulates are dissolved in, or flushed away by, a liquid within the fiber bed through use of an irrigated fiber bed or of a fogging spray of liquid such as water injected into the gas stream prior to the fiber bed.
Re-entrainment of collected liquid from the downstream surface of the fiber bed often causes problems. These problems can include any of the following individually or in combination; fouling of downstream process equipment, degradation of product purity, corrosion to ductwork and in some cases difficulty in achieving emission requirements. Re-entrainment in fiber bed separators can arise from two mechanisms. As the liquid drains down through the fiber bed and/or the downstream surface thereof, the moving gas stream can cause some of the draining liquid to break or bubble out of the descending liquid stream and become re-entrained in the gas stream as droplets. This problem is particularly severe at the bottom of a vertically disposed fiber bed since all of the liquid collected by the fiber bed necessarily drains to the bottom and from a practical standpoint because of gas phase drag on the liquid, out the downstream surface at the bottom of the fiber bed. At this disengagement point where the greatest cumulative drainage occurs, gas phase drag can cause bubbling, “spitting”, jetting or fragmentation of the draining liquid. As these bubbles break, large to sub-micron sized fragments or droplets are formed which are carried away by the moving gas stream as what is termed “bubble re-entrainment”.
The second re-entrainment mechanism termed “bed re-entrainment” occurs at gas bed velocities so high that gas phase drag on the draining liquid in the entire fiber bed on downstream discharge surfaces of the fiber bed causes bubbling, spitting, jetting and fragmentation into re-entrainment. Thus, in a given fiber bed and at a constant liquid loading, as bed velocity increases, a point is reached where bubble re-entrainment begins. This first occurs at the bottom of the fiber bed on the gas discharge surface of the collecting media. As the bed velocity is increased even further re-entrainment begins to occur at higher levels on the fiber bed until with only minor increases in velocity, re-entrainment is occurring from substantially the entire gas discharge surface of the fiber bed. This is typically referred to as a totally flooded condition.
Prior attempts have been made to prevent re-entrainment and to operate the fiber bed in a drier condition by removing collected liquid and soluble solids from the fiber bed. In one example a tubular fiber bed is formed of two or more shorter sections that are stacked one on top of the other to form the fiber bed. A metal plate is placed between adjacent sections in the stack to form a barrier against migration of liquid from one section into the next lower section and to cause liquid to flow radially away from the sections (and out of the fiber bed). However in this arrangement, it is possible for the liquid to move out of the fiber bed to the discharge (i.e., downstream) face of the fiber bed. At this location, the chances of the liquid becoming re-entrained increase. In addition, it is difficult to maintain the necessary gas seal between the fiber bed sections and the metal plate to avoid gas bypassing between the section and the adjoining plate.